Rubber composition

ABSTRACT

A rubber composition comprising (i) an incompatible polymer blend of at least two diene rubbers selected from rubbers containing a conjugated diene and, optionally, an aromatic vinyl monomer and forming two polymer phases (A) and (B), and (ii) 0.1 to 20 parts by weight, based upon 100 parts by weight of the total polymer component including the block copolymer, of a block copolymer having at least two mutually incompatible blocks (a) and (b), wherein the block (a) is compatible with the polymer phase (A) and incompatible with the polymer phase (B) and the block (b) is compatible with the polymer phase (B) and incompatible with the polymer phase (A), and composed of a conjugated diene and, optionally, an aromatic vinyl monomer, and wherein the molecular weights of the polymers forming the polymer phases (A) and (B) satisfy the specified equations (I) and (II) mentioned in the specification.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a rubber composition, morespecifically relates to a rubber composition suitable for use for a tiretread, sidewalls, or other rubber parts and having improved tensilestrength, elongation, and abrasion resistance.

[0003] 2. Description of the Related Art

[0004] In recent years, improvements in various types of performancehave been sought in the rubber compositions for automobile and othertires etc. Therefore, in tire tread rubber and the like, several typesof polymers have been used by blending together. However, when thesepolymers are incompatible with each other, phase separated interfacesare present. In most cases, these interfaces become starting points forbreakage and are believed to have a detrimental effect on the tensilestrength, tear strength, abrasion resistance, etc. However, in tire andother rubber products, since the special process of vulcanization isincluded, it is not possible to apply as is the molecular design of theblock copolymer for control of the phase structure as is normally donein rubber/resin blends or resin/resin blends. Therefore, the problem ofthe phase separation interface of rubber/rubber blends has not beensufficiently studied and no method for solving this problem had beenfound yet.

[0005] In the past, the decrease in the breaking strength due to theincompatibility of polymer blends obtained by blending block copolymershas not been sufficiently studied. Blending, into a blend of naturalrubber (NR)/polybutadiene rubber (BR), a small amount of a blockcopolymer of polybutadiene (BR) and polyisoprene (IR) has only beendescribed slightly in J. Apply. Polym. Sci., 49 (1993) and RCT. 66(1993). However, the compositions of the block copolymers used in thesereferences have insufficient compatibility with BR, and therefore, arenot satisfactory in performance for practical use. Further, attemptshave been made to add cis-BR into an incompatible polymer blend ofcis-BR/SBR so as to improve the abrasion resistance, but the wet brakingperformance is decreased, and therefore, there is a limit to the amountof addition of cis-BR and there were consequently problems in practicaluse. In addition, except for the proposals made by the inventors of thepresent invention (Japanese Unexamined Patent Publication (Kokai) Nos.7-188510, 8-134267, 8-193147, 8-193146, 8-193145, 8-283465, 8-302071,10-007844, and 10-036465), examples of blending a block polymer into arubber composition as a compatibilizing agent have not been known. Theprevious proposals by the present inventors did not clarify therelationship between the rubber component forming the matrix of therubber composition and the molecular weight of the block polymer added.Later study resulted in clarification of this point and the presentinvention has been completed.

SUMMARY OF THE INVENTION

[0006] Accordingly, the object of the present invention is to provide arubber composition capable of eliminating the, above-mentioned problemsin the prior art and improving the tensile strength, elongation,abrasion resistance, etc. thereof.

[0007] In accordance with the present invention, there is provided arubber composition comprising (i) an incompatible polymer blendcomprising at least two diene rubbers selected from the group consistingof rubbers containing at least one conjugated diene monomer andoptionally at least one aromatic vinyl monomer, such as natural rubber(NR), polyisoprene rubber (IR), polybutadiene rubber (BR),styrene-butadiene copolymer rubber (SBR), styrene-isoprene copolymerrubber (SIR) and styrene-isoprene-butadiene rubber (SIBR) and formingtwo incompatible polymer phases (A) and (B) and (ii) 0.1 to 20 parts byweight, based upon 100 parts by weight of the total polymer componentincluding the block copolymer, of a block copolymer having at least twomutually incompatible blocks (a) and (b), the block (a) being compatiblewith the polymer phase (A) and being incompatible with the polymer phase(B) and the block (b) being compatible with the polymer phase (B) andincompatible with the polymer phase (A), and comprising at least oneconjugated diene monomer (e.g., isoprene, butadiene) and, optionally, atleast one aromatic vinyl monomer (e.g., styrene), wherein the molecularweights of the polymers forming the polymer phases (A) and (B) satisfythe following equations (I) and (II):

S _(A) =Mw ₃₀(A)/Mw(a)≦1.2  (I)

S _(B) =Mw ₃₀(B)/Mw(b)≦1.2  (II)

[0008] wherein

[0009] Mw₃₀(A): molecular weight of the low molecular weight portion ofthe polymer forming the polymer phase (A),

[0010] Mw₃₀(B): molecular weight of the low molecular weight portion ofthe polymer forming the polymer phase (B),

[0011] Mw(a): weight average molecular weight of block (a) of blockcopolymer, and

[0012] Mw(b): weight average molecular weight of block (b), of blockcopolymer.

[0013] In accordance with the present invention, there is also provideda rubber composition, wherein 5 to 200 parts by weight, based upon 100parts by weight of the block copolymer, of a polymer (α) compatible withthe block (a) and the polymer phase (A) and/or a polymer (β) compatiblewith the block (b) and polymer phase (B) are further blended and theweight average molecular weights of the polymers (α) and (β) satisfy thefollowing equations (III) and (IV):

S _(α) =Mw(α)/Mw(a)≦1.2  (III)

S _(β) =Mw(β)/Mw(b)≦1.2  (IV)

[0014] wherein

[0015] Mw(α): weight average molecular weight of polymer (α),

[0016] Mw(β): weight average molecular weight of polymer (β),

[0017] Mw(a): weight average molecular weight of block (a) of blockcopolymer, and

[0018] Mw(b): weight average molecular weight of block (b) of blockcopolymer.

[0019] In accordance with the present invention, there is furtherprovided a rubber composition comprised of a block copolymer having atleast two mutually incompatible blocks (a) and (b) and comprising atleast one conjugated diene and, optionally, at least one aromatic vinylmonomer based upon 100 parts by weight of the same, 5 to 200 parts byweight of a polymer (α) compatible with the block (a) and/or a polymer(β) compatible with the block (b), the weight average molecular weightsof the polymers (α) and (β) satisfying the following equations (III) and(IV):

S _(α) =Mw(α)/Mw(a)≦1.2  (III)

S _(β) =Mw(β)/Mw(b)≦1.2  (IV)

[0020] wherein

[0021] Mw(α): weight average molecular weight of polymer (α),

[0022] Mw(β): weight average molecular weight of polymer (β),

[0023] Mw(a): weight average molecular weight of block (a) of blockcopolymer, and

[0024] Mw(b): weight average molecular weight of block (b) of blockcopolymer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present invention will be better understood from thedescription set forth below with reference to the accompanying drawings,wherein

[0026]FIG. 1 is a view of an example of a molecular weight distributioncurve (integrated molecular weight curve) of molecular weights measuredby GPC forming the basis for finding the molecular weights of the lowmolecular weight portions of the polymers of the polymer phases (A) and(B) of equations (I) and (II);

[0027] GPC Measurement Conditions

[0028] GPC: HLC-8020 made by Toso

[0029] Column: GMH-HR-H, 2

[0030] Temperature: 40° C.

[0031] Mobile phase: THF

[0032] Standard substance: 10 points used between standard polystyrene1000 to 10,000,000

[0033] Approximation method: By tertiary method.

[0034] Preparation of polymer sample: 50 mg of the polymer was dissolvedin 10 cc of THF. The mixture was stirred at room temperature for about168 hours so as to dissolve. This was then filtered by a 0.5 micronfilter (H25-5 made by Toso) to remove the insolubles. The result wasused as the sample. The amount injected into the GPC was made 400 μl.

[0035]FIG. 2 is a view of an integrated molecular weight curve obtainedby converting the molecular weight distribution curve of FIG. 1, whereinMw₃₀(A) and Mw₃₀(B) of equations (I) and (II) are found from themolecular weight of the cumulative area 30% as shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] The present inventors found that the above-mentioned object canbe achieved by compounding, into a rubber composition formed from atleast two rubber phases comprising at least two incompatible rubbers, ablock polymer comprising at least two incompatible blocks havingmolecular weights which are defined by two types of relationships withthe molecular weights of the rubbers forming the two rubber phases.

[0037] The tire rubber composition according to the present inventioncan be obtained by blending (i) an incompatible polymer blend of twopolymer phases (A) and (B) comprising at least two types of incompatiblerubbers of NR, IR, BR, SBR, SIR, and SIBR, preferably in the weightratio of (A)/(B) of 90/10 to 10/90, more preferably 85/15 to 15/85) and(ii) 0.1 to 20 parts by weight, preferably 1 to 15 parts by weight,based upon 100 parts by weight of the entire rubber component includinga block copolymer, of the block copolymer having at least two blockscomprising monomers selected from isoprene, butadiene, and styrene,wherein the blocks (a) and (b) are mutually incompatible, the block (a)is compatible with the polymer phase (A) and incompatible with thepolymer phase (B), and the block (b) is compatible with the polymerphase (B) and incompatible with the polymer phase (A), preferably theweight ratio of (a)/(b) of 80/20 to 20/80, more preferably 60/40 to40/60, and wherein the molecular weights of the polymers forming thepolymer phases (A) and (B) satisfy the equations (I) and (II). Note thatSA and SB preferably are 0.1 to 1.2, more preferably 0.3 to 1.0.

[0038] If the value of S_(A) and the value of S_(B) are more than 1.2,the block copolymer added while mixing the rubber component does notdisperse well and the effect as a compatibilizing agent cannot besufficiently exhibited.

[0039] Note that the molecular weights of the low molecular weightportions of the polymers forming the polymer phases (A) and (B) meanthose found as values of molecular weights (i.e., Mw₃₀(A) and Mw₃₀(B))corresponding to 30% of the cumulative area when converting the curve ofthe distribution of the molecular weight measured by GPC such as shownin FIG. 1 to the integrated molecular weight curve, as shown in FIG. 2.The GPC is measured by, for example, dissolving the polymer sample intoTHF, removing the insoluble gel component by a 0.5 micron filter, thencalculating the molecular weight by an equation obtained from the amountof elution of standard polystyrene.

[0040] Further, it is possible to previously blend a block copolymerhaving at least two mutually incompatible blocks (a) and (b) andcomprising a conjugated diene and/or aromatic vinyl monomer with apolymer (α) compatible with the block (a) and the polymer phase (A)and/or a polymer (β) compatible with the block (b) and the polymer phase(B), which satisfies the following equations (III) and (IV):

S _(α) =Mw(α)/Mw(a)≦1.2  (III)

S _(β) =Mw(β)/Mw(b)≦1.2  (IV)

[0041] wherein

[0042] Mw(α): weight average molecular weight of polymer (α),

[0043] Mw(β): weight average molecular weight of polymer (β),

[0044] Mw(a): weight average molecular weight of block (a) of blockcopolymer, and

[0045] Mw(b): weight average molecular weight of block (b) of blockcopolymer,

[0046] so as to improve the dispersion of the block copolymer duringmixing of the rubber and obtain the better mechanical strength.

[0047] Note that S_(α) and S_(β) are preferably 0.1 to 1.2, morepreferably 0.3 to 1.0.

[0048] In particular, (α) should be added when the low molecular weightportion of the polymer forming the polymer phase (A) is small and (β)should be added when the low molecular weight portion of the polymerforming the polymer phase (B) is small. Therefore, even when the aboveequations (I) and/or (II) are not satisfied, it is possible to obtainthe effect of the present invention by blending in the polymer (α) or(β).

[0049] Here, as the polymer (α) or (β), rubbers such as IR, BR, SBR,SIBR having a suitable molecular weight are preferable, but it is notlimited to a rubber so long as the object of improving the dispersion atthe time of mixing the block polymer is achieved without impairing thevulcanized physical properties of the rubber composition finallyobtained. Other polymers may also be used.

[0050] The amount of the polymer (α) or (β) blended should be 5 to 200parts by weight, preferably 20 to 100 parts by weight, based upon 100parts by weight of the block copolymer. If the amount is less than 5parts by weight, the anticipated effect is not manifested, while if morethan 200 parts by weight, the elasticity or mechanical strength isdecreased, and therefore, there is a detrimental effect on the physicalproperties or the Mooney viscosity of the starting rubber is decreasedand handling becomes difficult.

[0051] The process of production of the block copolymer used in thepresent invention is not particularly limited, but, for example, thismay be produced by polymerizing isoprene, butadiene, or styrene monomersin a hydrocarbon solvent using an organoactive metal as an initiator. Asthe organoactive metal, for example, an anionic polymerizableorganoactive metal such as an organoalkali metal compound, organoalkaliearth metal compound, or organolanthanoid based rare earth metalcompound may be mentioned. Among these, an organoalkali metal compoundis particularly preferable.

[0052] According to the present invention, it is possible to furtherblend low molecular weight polymers (for example, IR, BR, SBR, or SIBR)as parts of the polymers forming the polymer phases so as to satisfy theabove equations (I) and (II). The amounts of the low molecular weightpolymers blended are preferably 1 to 50 parts by weight, based upon 100parts by weight of the rubber component, as a whole. If the amountsblended are too great, this leads to the decrease in the tensilestrength etc., and therefore, this is not preferred.

[0053] The incompatible polymer blend comprising the polymer phases (A)and (B) used in the present invention is not particularly limited solong as two or more types of polymers selected from polymers containingconjugated dienes and/or aromatic vinyl monomers such as NR, IR, BR,SBR, are selected and constitute two incompatible polymer phases (A) and(B). Further, the block copolymer comprising the blocks (a) and (b) usedin the present invention may be made any polymer provided with the aboveconditions. For example, a BR block, SBR block, IR block, SIR (i.e.,styrene isoprene rubber) block, BIR (i.e., butadiene isoprene) block,SBIR (i.e., styrene butadiene isoprene) block, etc. may be suitablycombined for use.

[0054] Representative examples of combinations of such incompatiblepolymers and block copolymers are as follows: TABLE I Matrix polymer(A)/(B) Block copolymer ((a)/(b)) NR/SBR (wherein, amount of IR/SBR(amount of vinyl of vinyl of Bd part is not Bd part not more than aboutmore than about 60 mol %) 60 mol %) or SBR/SBR (amount of St about 20%by weight, amount of vinyl of Bd part about 70 mol %) NR/BR (cis contentnot less IR/SBR (amount of St about than 90 mol %) 20% by weight, amountof vinyl of Bd part about 50 mol %)

[0055] Of course, the present invention is not limited to the aboveexamples.

[0056] The rubber composition according to the present invention maysuitably use various conventional additives according to itsapplication, for example, various reinforcing fillers generally used inthe prior art such as carbon black and silica, softeners, antioxidant,wax, resin, vulcanization agent, vulcanization accelerator,vulcanization accelerator activator, etc. Further, blowing agent, lowmoisture plasticizer, short fibers, etc. may be used.

[0057] In blending the rubber composition according to the presentinvention, it is preferable to first mix the rubber (i.e., matrix rubberand block copolymer) and the additives other than, for example,vulcanization agent and vulcanization accelerator according to anordinary method, then blend them. Of course, even if some of theseingredients are separately mixed, the resultant mixture, needless tosay, falls in the technical scope of the present invention so long asthe object of the present invention is not impaired. Further, theblending may be carried out in any means used in the past.

[0058] The rubber composition of the present invention may be vulcanizedby a general method. The amount of the above additives blended may bethe general amounts. Further, the vulcanization conditions may be madethe general conditions.

EXAMPLES

[0059] The present invention will be further illustrated with referenceto Examples, but the present invention is of course by no means limitedin scope by these Examples.

Standard Examples 1 to 6, Examples 1 to 12, and Comparative Examples 1to 7

[0060] The ingredients of the formulations of Tables II to IV, Table V,and Table VI (parts by weight) (wherein the characteristics of thepolymers used as the phase (A) and phase (B) are shown in Table VII, thecharacteristics of the block polymers are shown in Table VIII, and thecharacteristics of the polymers (α) and (β) are shown in Table IX) weremixed in 1.5 liter Bambury mixers for 4 minutes, then the vulcanizationaccelerators and sulfur were mixed with the mixtures by 8-inch test-useroll mill to obtain the rubber compositions. These rubber compositionswere press vulcanized at 160° C. for 20 minutes to prepare the desiredtest pieces which were then subjected to various tests and measured inphysical properties. The physical properties of the vulcanates obtainedwere as shown in Tables II, III and IV.

[0061] Mixing Method

[0062] The mixing methods used in the Examples and the ComparativeExamples were all according to the following mixing specifications:

[0063] 1) Rotor speed: 60 rpm

[0064] 2) Temperature adjustment: 50° C.

[0065] 3) Charging specifications:

[0066] 0′ . . . rubber ingredients (matrix rubber, block copolymer)

[0067] 1′ . . . carbon black in half amount, zinc white, stearic acid

[0068] 2′30″ . . . carbon black in half amount, antioxidant, wax

[0069] 3′30″ . . . raising and lowering of ram (cleaning ram portion)

[0070] 4′00″ . . . discharge

[0071] The “yes” in the compatibility section of Tables II, III and IVindicates a compatible relationship, while the “no” indicates anincompatible relationship. TABLE II Standard Comp. Ex. Comp. Ex. 1 1 Ex.2 Ex. 1 phase (A) polymer NR-2 50 45 45 45 phase (B) polymer SBR 50 4545 45 Block polymer BP-1 — 10 — — BP-2 — — 10 — BP-3 — — — 10Compatibility Block (a) ⇄ block (b) — No No No Block (a) ⇄ phase (A) —Yes Yes Yes polymer Block (a) ⇄ phase (B) — No No No polymer Block (b) ⇄phase (A) — No No No polymer Block (b) ⇄ phase (B) — Yes Yes Yes polymerRelation with molecular weight S_(A) = Mw₃₀ (A)/Mw (a) — 1.8 1.7 0.8S_(B) = Mw₃₀ (B)/Mw (b) — 1.2 0.6 0.6 Physical properties of rubbercomposition Tensile strength (MPa) 23.2 23.5 24.0 26.3 Elongation (%)370 378 380 418 Abrasion resistance 100 102 101 120 index (index)

[0072] TABLE III Standard Standard Standard Ex. 2 Ex. 3 Ex. 4 phase (A)polymer NR-1 80 — — NR-2 — 80 — NR-3 — — 80 phase (B) polymer BR 20 2020 Physical properties of rubber composition Tensile strength (MPa) 29.928.1 26.6 Elongation (%) 568 578 579 Abrasion resistance index 100 100100 (index) Times to breakage in 2195900 2320300 2342100 fatigue testComp. Ex. 3 Comp. Ex. 4 Ex. 2 phase (A) polymer NR-2 78 78 78 phase (B)polymer BR 19 19 19 Block polymer BP-4 3 — — BP-5 — 3 — BP-6 — — 3Compatibility Block (a) ⇄ block (b) No No No Block (a) ⇄ phase (A)polymer Yes Yes Yes Block (a) ⇄ phase (B) polymer No No No Block (b) ⇄phase (A) polymer No No No Block (b) ⇄ phase (B) polymer Yes Yes YesRelation with molecular weight S_(A) = Mw₃₀ (A)/Mw (a) 1.7 1.7 0.9 S_(B)= Mw₃₀ (B)/Mw (b) 0.7 0.3 0.3 Physical properties of rubber compositionTensile strength (MPa) 28.4 28.3 30.6 Elongation (%) 581 580 586Abrasion resistance index 101 102 106 (index) Times to breakage infatigue 2310000 2298700 3212400 test Comp. Ex. 5 Ex. 3 phase (A) polymerNR-1 78 — NR-3 — 78 phase (B) polymer BR 19 19 Block polymer BP-5 — 3BP-6 3 — Compatibility Block (a) ⇄ block (b) No No Block (a) ⇄ phase (A)polymer Yes Yes Block (a) ⇄ phase (B) polymer No No Block (b) ⇄ phase(A) polymer No No Block (b) ⇄ phase (B) polymer Yes Yes Relation withmolecular weight S_(A) = Mw₃₀ (A)/Mw (a) 1.3 1.2 S_(B) = Mw₃₀ (B)/Mw (b)0.3 0.3 Physical properties of rubber composition Tensile strength (MPa)29.9 29.1 Elongation (%) 572 599 Abrasion resistance index (index) 102107 Times to breakage in fatigue test 2188800 3400200

[0073] TABLE IV Standard Comp. Ex. Ex. 5 6 Ex. 4 Ex. 5 phase (A) polymerNR-1 60 58 55 57.85 NR-2 — — — — phase (B) polymer BR 40 39 39 39 Blockpolymer BP-7 — 3 3 3 Polymer (α) α-1 — — 3 — α-2 — — — 0.15 α-3 — — — —α-4 — — — — Polymer (β) β-1 — — — — Compatibility Block (a) ⇄ block (b)— No No No Block (a) ⇄ phase (A) — Yes Yes Yes polymer Block (a) ⇄ phase(B) — No No No polymer Block (b) ⇄ phase (A) — No No No polymer Block(b) ⇄ phase (B) — Yes Yes Yes polymer Polymer (α) ⇄ block (a) — — YesYes Polymer (α) ⇄ phase — — Yes Yes (A) polymer Polymer (β) ⇄ block (b)— — — — Polymer (β) ⇄ phase (B) — — — — polymer Relation with molecularweight S_(A) = Mw₃₀ (A)/Mw (a) — 2.2 2.2 2.2 S_(B) = Mw₃₀ (B)/Mw (b) —0.3 0.3 0.3 S_(α) = Mw (α)/Mw (a) — — 0.7 1 S_(β) = Mw (β)/Mw (b) — — —— Rate of polymers (α) and — — 100 5 (β) added/wt % (to block polymer)Physical properties of rubber composition Tensile strength (MPa) 27.928.3 29.2 28.7 Elongation (%) 550 560 588 575 Abrasion resistance 100102 110 103 index (index) Comp. Ex. 6 Ex. 7 Ex. 8 Ex. 7 phase (A)polymer NR-1 55 52 55 55 NR-2 — — — — phase (B) polymer BR 39 39 39 39Block polymer BP-7 3 3 3 3 Polymer (α) α-1 — — — — α-2 3 6 α-3 — — 3 —α-4 — — — 3 Polymer (β) β-1 — — — — Compatibility Block (a) ⇄ block (b)No No No No Block (a) ⇄ phase (A) Yes Yes Yes Yes polymer Block (a) ⇄phase (B) No No No No polymer Block (b) ⇄ phase (A) No No No No polymerBlock (b) ⇄ phase (B) Yes Yes Yes Yes polymer Polymer (α) ⇄ block (a)Yes Yes Yes Yes Polymer (α) ⇄ phase Yes Yes Yes Yes (A) polymer Polymer(β) ⇄ block (b) — — — — Polymer (β) ⇄ phase (B) — — — — polymer Relationwith molecular weight S_(A) = Mw₃₀ (A)/Mw (a) 2.2 2.2 2.2 2.2 S_(B) =Mw₃₀ (B)/Mw (b) 0.3 0.3 0.3 0.3 S_(α) = Mw (α)/Mw (a) 1 1 1.2 1.4 S_(β)= Mw (β)/Mw (b) — — — — Rate of polymers (α) and 100 200 100 100 (β)added/wt % (to block polymer) Physical properties of rubber compositionTensile strength (MPa) 29.7 28.5 28.8 28.4 Elongation (%) 585 588 577562 Abrasion resistance 108 103 105 102 index (index) Stand- ard Ex. Ex.9 Ex. 10 6 Ex. 11 Ex. 12 phase (A) polymer NR-1 58 56 — — — NR-2 — — 6058 57.4 phase (B) polymer BR 38 38 40 39 39 Block polymer BP-6 — — — 3 3Block polymer BP-7 3 3 — — — Polymer (α) α-1 — — — — — α-2 — 2 — — 0.6α-3 — — — — — α-4 — — — — — Polymer (β) β-1 1 1 — — — CompatibilityBlock (a) ⇄ block (b) No No — No No Block (a) ⇄ phase (A) Yes Yes — YesYes polymer Block (a) ⇄ phase (B) No No — No No polymer Block (b) ⇄phase (A) No No — No No polymer Block (b) ⇄ phase (B) Yes Yes — Yes Yespolymer Polymer (α) ⇄ block (a) — Yes — — Yes Polymer (α) ⇄ phase — Yes— — Yes (A) polymer Polymer (β) ⇄ block (b) Yes Yes — — — Polymer (β) ⇄phase (B) Yes Yes — — — polymer Relation with molecular weight S_(A) =Mw₃₀ (A)/Mw (a) 2.2 2.2 — 0.9 0.9 S_(B) = Mw₃₀ (B)/Mw (b) 0.3 0.3 — 0.30.3 S_(α) = Mw (α)/Mw (a) — 1.0 — — 0.6 S_(β) = Mw (β)/Mw (b) 0.3 0.3 —— — Rate of polymers (α) and 33 100 — — 20 (β) added/wt % (to blockpolymer) Physical properties of rubber composition Tensile strength(MPa) 28.4 30 26.8 27.7 29.8 Elongation (%) 570 590 520 566 602 Abrasionresistance 103 107 100 101 111 index (index)

[0074] TABLE V NR/SBR Blend Formulation (Parts by Weight) Rubbercomponent 100 Carbon black (N339)*1 50 Zinc white 3 Stearic acid 2Antioxidant (6C)*2 3 Wax 2 Vulcanization accelerator (NS)*3 1 Sulfur 1.7

[0075] TABLE VI NR/BR Blend Formulation (Parts by Weight) Rubbercomponent 100 Carbon black (N110)*1 50 Zinc white 5 Stearic acid 2Antioxidant (6C)*2 3 Vulcanization accelerator (NS)*3 1.2 Sulfur 1

[0076] TABLE VII Characteristics of Polymers Used as Phase (A) and Phase(B) Overall (MW) 30% (Mw) NR-1*1 7.57 × 10⁵ 3.9 × 10⁵ NR-2*2 1.19 × 10⁶2.6 × 10⁵ NR-3*3 4.65 × 10⁵ 1.8 × 10⁵ SBR*4 3.72 × 10⁵ 1.9 × 10⁵ BR*53.51 × 10⁵ 1.1 × 10⁵

[0077] TABLE VIII Characteristics of Block Polymer Block (a) Block (b)Microstructure Mw Microstructure Mw BP-1 Polyisoprene 1.48 × SBR (St =18 wt %, 1.61 × (cis/trans/vn = 10⁵ Vn = 11 mol %) 10⁵ 77/16/7) BP-2Polyisoprene 1.52 × SBR (St = 18 wt %, 3.12 × (cis/trans/vn = 10⁵ Vn =11 mol %) 10⁵ 77/16/7) BP-3 Polyisoprene 3.10 × SBR (St = 18 wt %, 3.21× (cis/trans/vn = 10⁵ Vn = 11 mol %) 10⁵ 77/16/7) BP-4 Polyisoprene 1.51× SBR (St = 19 wt %, 1.47 × (cis/trans/vn = 10⁵ Vn = 46 mol %) 10⁵77/16/7) BP-5 Polyisoprene 1.49 × SBR (St = 19 wt %, 3.22 ×(cis/trans/vn = 10⁵ Vn = 46 mol %) 10⁵ 77/16/7) BP-6 Polyisoprene 3.01 ×SBR (St = 19 wt %, 3.21 × (cis/trans/vn = 10⁵ Vn = 46 mol %) 10⁵77/16/7) BP-7 Polyisoprene 1.80 × SBR (St = 19 wt %, 3.22 ×(cis/trans/vn = 10⁵ Vn = 46 mol %) 10⁵ 77/16/7)

[0078] TABLE IX Characteristics of Polymers Used as (α) and (β)Microstructure Mw α-1 Polyisoprene (cis/trans/vn = 77/16/7) 1.2 × 10⁵α-2 Polyisoprene (cis/trans/vn = 77/16/7) 1.8 × 10⁵ α-3 Polyisoprene(cis/trans/vn = 77/16/7) 2.2 × 10⁵ α-4 Polyisoprene (cis/trans/vn =77/16/7) 2.5 × 10⁵ β-1 SBR (St = 19 wt %, Vn = 46 mol %) 1.0 × 10⁵

[0079] The physical properties evaluated in the above Examples weremeasured by the following methods:

[0080] Tensile strength (Mpa): Measured according to JIS K6251.

[0081] Elongation (%): Measured according to JIS K6251.

[0082] Abrasion resistance test: Measured using a Lambourn abrasiontester at conditions of a slip rate of 25% and a load of 5 kg. Theresults are shown indexed to the formulation of the correspondingStandard Examples as 100 (abrasion resistance index). The larger thefigure, the better the abrasion resistance shown.

[0083] Times to breakage in fatigue test: Shown by number of times tobreakage of a JIS No. 3 dumbbell shaped sample after being givenrepeated deformation at a cycle rate of 400 rpm at an elongation stressof 100% (average for four tests).

[0084] The incompatibility of polymers was judged by the followingmethod:

[0085] 1) The incompatibility of the polymer phases (A) and (B) of thepolymer blend was judged by vulcanizing the polymer blend, preparingultrathin slice samples by the freezing method, then dyeing these in agas phase with a benzene solution of osmium tetraoxide at roomtemperature for about 15 hours. The presence of phase (a) separatedstructure was examined by observation through a transmission typeelectron microscope at magnifications of about 5000 to 10,000.

[0086] 2) The incompatibility of the blocks (a) and (b) of the blockcopolymer was judged by preparing samples in the same way as above fromthe block copolymer in the unvulcanized state, then observing themthrough a transmission type electron microscope at a magnification ofabout 60,000 to examine the presence of a phase separated structure.

[0087] 3) The incompatibility of the blocks of the block copolymer andthe polymer phases of the polymer blend was judged by separatingpolymerizing and preparing the polymers corresponding to the polymersconstituting the blocks, mixing these with the matrix polymers,vulcanizing them, then proceeding in the same way as above to preparesamples for observation through an electron microscope and observingthese at magnifications of about 5000 to 10,000 to examine the presenceof phase separated structures.

[0088] In addition, the compatibilities and incompatibilities may bedecided by judging the presence of bimodal or not from the temperaturedependence curve of tan δ or by judging the presence of plurality ofglass transition temperatures or not of the blend polymers can beobserved by DSC measurement and further may be judged by an opticalmicroscope if the phase separated structure reaches as much as severaldozen microns. Among these, the above direct observation by an electronmicroscope is the most sensitive method.

[0089] As explained above and as shown in Examples 1 to 12, the rubbercompositions according to the present invention are improved inmechanical strength such as tensile strength, elongation, abrasionresistance, and fatigue resistance compared with the rubber compositionsof Comparative Examples 1 to 7.

1. A method for producing a rubber composition having an improvedabrasion resistance and tensile strength by controlling a molecularweight distribution of a rubber composition comprising (i) anincompatible polymer blend comprising at least two diene rubbersselected from the group consisting of rubbers containing at least oneconjugated diene monomer and, optionally, at least one aromatic vinylmonomer and forming two incompatible polymer phases (A) and (B) and (ii)0.1 to 20 parts by weight, based upon 100 parts by weight of the totalpolymer component including the block copolymer, of a block copolymerhaving at least two mutually incompatible blocks (a) and (b) in whichthe block (a) is compatible with the polymer phase (A) and incompatiblewith the polymer phase (B) and the block (b) is compatible with thepolymer phase (B) and incompatible with the polymer phase (A), andcomprising at least one conjugated diene monomer and, optionally, atleast one aromatic vinyl monomer, such that the polymers forming thepolymer phases (A) and (B) satisfy the following equations (I) and (II):Mw ₃₀(A)/Mw(a)≦1.2  (I) Mw ₃₀(B)/Mw(b)≦1.2  (II) wherein Mw₃₀(A): avalue of molecular weight corresponding to 30% of the cumulative areawhen converting the curve of the distribution of the molecular weightmeasured by GPC to the integrated molecular weight curve of the polymerforming the polymer phase (A), Mw30(B): a value of molecular weightcorresponding to 30% of the cumulative area when converting the curve ofthe distribution of the molecular weight measured by GPC to theintegrated molecular weight curve of the polymer forming the polymerphase (B), Mw(a): weight average molecular weight of block (a) of blockcopolymer, and Mw(b): weight average molecular weight of block (b) ofblock copolymer.
 2. A method as claimed in claim 1, wherein (iii) 5 to200 parts by weight, based upon 100 parts by weight of the blockcopolymer of polymer (α) compatible with the block (a) and the polymerphase (A) and/or polymer (β) compatible with the block (b) and polymerphase (B) are further blended and the weight average molecular weightsof the polymer (α) and (β) satisfy the following equations (III) and(IV): S _(α) =Mw(α)/Mw(a)≦1.2  (III) S _(β) =Mw(β)/Mw(b)≦1.2  (IV)wherein Mw(α): weight average molecular weight of polymer (α), Mw(β):weight average molecular weight of polymer (β) Mw(a): weight averagemolecular weight of block (a) of block copolymer, and Mw(b): weightaverage molecular weight of block (b) of block copolymer.
 3. A method asclaimed in claim 1, wherein said diene rubbers are NR, IR, BR, SBR, SIRand SIBR.
 4. A method as claimed in claim 3, wherein a weight ratio ofpolymer phase (A)/polymer phase (B) is 90/10 to 10/90.
 5. A method asclaimed in claim 1, wherein said block copolymer contains at least twoblocks selected from the group consisting of BR block, SBR block, IRblock, SIR block, BIR block and SBIR block.
 6. A method as claimed inclaim 5, wherein a weight ratio of block (a)/block (b) is 80/20 to20/80.
 7. A method as claimed in claim 2, wherein said polymers (α) and(β) are selected from IR, BR, SBR and SIBR.